[WebAssembly] Update function signature mismatch error message. NFC.

We don't start our error messages with capital letters.

Split out from https://reviews.llvm.org/D48394

Differential Revision: https://reviews.llvm.org/D48400

llvm-svn: 335186
diff --git a/lld/wasm/SymbolTable.cpp b/lld/wasm/SymbolTable.cpp
index 6cc7c36..1180cfc 100644
--- a/lld/wasm/SymbolTable.cpp
+++ b/lld/wasm/SymbolTable.cpp
@@ -116,7 +116,7 @@
 
   const WasmSignature *OldSig = ExistingFunction->getFunctionType();
   if (OldSig && NewSig && *NewSig != *OldSig) {
-    warn("Function type mismatch: " + Existing->getName() +
+    warn("function signature mismatch: " + Existing->getName() +
          "\n>>> defined as " + toString(*OldSig) + " in " +
          toString(Existing->getFile()) + "\n>>> defined as " +
          toString(*NewSig) + " in " + toString(File));