layers: Fix git send-email instructions in README

This ensures that cut-n-paste of this command from README will work

Signed-off-by: Khem Raj <raj.khem@gmail.com>

meta-filesystems/README

@@ -30,7 +30,7 @@ When sending single patches, please use something like:
 
    git send-email -1 -M \
         --to openembedded-devel@lists.openembedded.org \
-        --subject-prefix=meta-filesystems][PATCH
+        --subject-prefix='meta-filesystems][PATCH'
 
 
 Table of Contents
@@ -99,4 +99,3 @@ II. Misc
 
   --- xfsprogs ---
   It provides XFS filesystem utilities.
-