Fix typo

Author: the-msng-link

Sources/protoc-gen-swift/FileGenerator.swift

@@ -181,7 +181,7 @@ class FileGenerator {
             "// If the compiler emits an error on this type, it is because this file\n",
             "// was generated by a version of the `protoc` Swift plug-in that is\n",
             "// incompatible with the version of SwiftProtobuf to which you are linking.\n",
-            "// Please ensure that your are building against the same version of the API\n",
+            "// Please ensure that you are building against the same version of the API\n",
             "// that was used to generate this file.\n",
             "fileprivate struct _GeneratedWithProtocGenSwiftVersion: SwiftProtobuf.ProtobufAPIVersionCheck {\n")
         p.indent()