Update build-ref.md
Very little typo
Closes
@@ -49,7 +49,7 @@ can be used to produce output artifacts.
A package is defined as a directory containing a file named `BUILD`
(or `BUILD.bazel`). A package includes all files in its directory, plus
all subdirectories beneath it, except those which themselves contain a
-`BUILD` file. From this definition, no file or directory my be a part of
+`BUILD` file. From this definition, no file or directory may be a part of
two different packages.
For example, in the following directory tree
