Fix typos in unstable-book

Author: xzmeng

src/doc/unstable-book/src/compiler-flags/path-options.md

@@ -1,6 +1,6 @@
 # `--print` Options
 
-The behavior of the `--print` flag can be modified by optionally be specifiying a filepath
+The behavior of the `--print` flag can be modified by optionally be specifying a filepath
 for each requested information kind, in the format `--print KIND=PATH`, just like for
 `--emit`. When a path is specified, information will be written there instead of to stdout.