Fix precedence of code owners (#6)

Later lines in a CODEOWNERS file take precedence over earlier lines.
Previously, we put the two code owners on separate lines with identical
wildcard patterns. This caused only the second code owner to be
requested for review. This update puts both code owners on the same
line, indicating co-ownership.

.github/CODEOWNERS

@@ -1,2 +1 @@
-* @nemo794
-* @gmgunter
+* @nemo794 @gmgunter