doc: fix a typo and clarify a sentence

I noticed that git-merge-base was unlikely to actually be a git command,
and tried it in my shell. Seeing that it doesn't work, I cleaned up two
places in the docs where it appears.

Signed-off-by: Mihir Mehta <mihir@cs.utexas.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-diff.txt

@@ -72,10 +72,10 @@ two blob objects, or changes between two files on disk.
 	This form is to view the changes on the branch containing
 	and up to the second <commit>, starting at a common ancestor
 	of both <commit>.  "git diff A\...B" is equivalent to
-	"git diff $(git-merge-base A B) B".  You can omit any one
+	"git diff $(git merge-base A B) B".  You can omit any one
 	of <commit>, which has the same effect as using HEAD instead.
 
-Just in case if you are doing something exotic, it should be
+Just in case you are doing something exotic, it should be
 noted that all of the <commit> in the above description, except
 in the last two forms that use ".." notations, can be any
 <tree>.

Documentation/howto/update-hook-example.txt

@@ -80,7 +80,7 @@ case "$1" in
       info "The branch '$1' is new..."
     else
       # updating -- make sure it is a fast-forward
-      mb=$(git-merge-base "$2" "$3")
+      mb=$(git merge-base "$2" "$3")
       case "$mb,$2" in
         "$2,$mb") info "Update is fast-forward" ;;
 	*)	  noff=y; info "This is not a fast-forward update.";;