Update README.md (#5282)

a -> an

README.md

@@ -374,7 +374,7 @@ Each individual file size is limited to the volume size.
 
 ### Saving memory ###
 
-All file meta information stored on an volume server is readable from memory without disk access. Each file takes just a 16-byte map entry of <64bit key, 32bit offset, 32bit size>. Of course, each map entry has its own space cost for the map. But usually the disk space runs out before the memory does.
+All file meta information stored on a volume server is readable from memory without disk access. Each file takes just a 16-byte map entry of <64bit key, 32bit offset, 32bit size>. Of course, each map entry has its own space cost for the map. But usually the disk space runs out before the memory does.
 
 ### Tiered Storage to the cloud ###