- Fixed typo in buck2 docs

Summary: - Fixed typo in buck2 public docs that I noticed while reading

Reviewed By: abashyam

Differential Revision: D53170262

fbshipit-source-id: fc6ea3acd113eed9c7c5b7a8510732d651645119

docs/concepts/build_target.md

@@ -88,7 +88,7 @@ is flexible when parsing build targets on the command-line. Specifically, the
 leading `//` is optional on the command line, so the above could be:
 
 ```
-buck build java/com/facebook/share:share
+buck2 build java/com/facebook/share:share
 ```
 
 Also, if there is a forward slash before the colon, it is ignored, so this could