update path of profile.proto file (#490)

doc/README.md

 profile.proto is a protocol buffer that describes a set of callstacks
 and symbolization information. A common usage is to represent a set of
 sampled callstacks from statistical profiling. The format is
-described on the src/proto/profile.proto file. For details on protocol
+described on the proto/profile.proto file. For details on protocol
 buffers, see https://developers.google.com/protocol-buffers
 
 Profiles can be read from a local file, or over http. Multiple