Fix javadoc typos in JGit API

There were some small errors which made it
difficult to read the JavaDoc.

Change-Id: Ib3b34353465162adebaca3514d596d0edf5aea51
Signed-off-by: Stefan Lay <stefan.lay@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/api/ConcurrentRefUpdateException.java

 import org.eclipse.jgit.lib.RefUpdate;
 
 /**
- * Exception thrown when a command want's to update a ref but failed because
+ * Exception thrown when a command wants to update a ref but failed because
  * another process is accessing (or even also updating) the ref.
  *
  * @see RefUpdate.Result#LOCK_FAILURE

org.eclipse.jgit/src/org/eclipse/jgit/api/Git.java

 	private final Repository repo;
 
 	/**
-	 * Constructs a new {@link Git} class which can interact with the specified
+	 * Constructs a new {@link Git} object which can interact with the specified
 	 * git repository. All command classes returned by methods of this class
 	 * will always interact with this git repository.
 	 *
 	}
 
 	/**
-	 * Returns a command class to execute a {@code Commit} command
+	 * Returns a command object to execute a {@code Commit} command
 	 *
 	 * @see <a
 	 *      href="http://www.kernel.org/pub/software/scm/git/docs/git-commit.html"
 	}
 
 	/**
-	 * Returns a command class to execute a {@code Log} command
+	 * Returns a command object to execute a {@code Log} command
 	 *
 	 * @see <a
 	 *      href="http://www.kernel.org/pub/software/scm/git/docs/git-log.html"
 	}
 
 	/**
-	 * Returns a command class to execute a {@code Merge} command
+	 * Returns a command object to execute a {@code Merge} command
 	 *
 	 * @see <a
 	 *      href="http://www.kernel.org/pub/software/scm/git/docs/git-merge.html"
 	}
 
 	/**
-	 * @return the git repository this class is interacting with
+	 * @return the git repository this instance is interacting with
 	 */
 	public Repository getRepository() {
 		return repo;

org.eclipse.jgit/src/org/eclipse/jgit/api/NoMessageException.java

 package org.eclipse.jgit.api;
 
 /**
- * Exception thrown when the options given to a command doesn't include a
+ * Exception thrown when the options given to a command don't include a
  * specification of a message text (e.g. a commit was called without explicitly
  * specifying a commit message (or other options telling where to take the
  * message from.