Minor readme fix

[trueneu]

Minor typos fixes in README.md:

:protoc-source-paths -> :proto-source-paths

$ lein protoc --help
Compiles Google Protocol Buffer proto files to Java Sources.

  The following options are available and should be configured in the
  project.clj:

    :protoc-version     :: the Protocol Buffers Compiler version to use.

    :proto-source-paths :: vector of absolute paths or paths relative to
                           the project root that contain the .proto files
                           to be compiled. Defaults to `["src/proto"]`

    :proto-target-path  :: the absolute path or path relative to the project
                           root where the sources should be generated. Defaults
                           to `${target-path}/generated-sources/protobuf`

    :protoc-grpc        :: true (or empty map) to generate interfaces for gRPC
                           service definitions with default settings. Can
                           optionally provide a map with the following configs:
                             :version     - version number for gRPC codegen.
                             :target-path - absolute path or path relative to
                                            the project root where the sources
                                            should be generated. Defaults to
                                            the `:proto-target-path`
                           Defaults to `false`.

    :protoc-timeout     :: timeout value in seconds for the compilation process
                           Defaults to 60

and src/protoc -> src/proto, to make it consistent

[clj-grpc "0.1.0"] -> [clj-grpc "0.1.2"]