Fix typos

Author: liufengyun

compiler/test/dotty/tools/vulpix/FileDiff.scala

@@ -9,7 +9,7 @@ object FileDiff {
       s"""Test output dumped in: $actualFile
           |  See diff of the checkfile (`brew install icdiff` for colored diff)
           |    > diff $expectFile $actualFile
-          |  Replace checkfile with current output output
+          |  Replace checkfile with current output
           |    > mv $actualFile $expectFile
       """.stripMargin
 

docs/docs/contributing/testing.md

@@ -69,7 +69,7 @@ If the actual output mismatches the expected output, the test framework will dum
 Test output dumped in: tests/playground/neg/Sample.check.out
   See diff of the checkfile
     > diff tests/playground/neg/Sample.check tests/playground/neg/Sample.check.out
-  Replace checkfile with current output output
+  Replace checkfile with current output
     > mv tests/playground/neg/Sample.check.out tests/playground/neg/Sample.check
 ```
 
@@ -132,5 +132,5 @@ with `with-compiler` in their name.
  $ sbt
  > testCompilation --from-tasty
  ```
- 
+
  This mode can be run under `dotty-compiler-bootstrapped/testCompilation` to test on a bootstrapped Dotty compiler.