[flang][driver] Fix spelling (nfc)

Merely making sure that we use consistent spelling.

Author: banach-space

flang/include/flang/Frontend/CompilerInvocation.h

@@ -122,7 +122,7 @@ class CompilerInvocation : public CompilerInvocationBase {
   const Fortran::parser::Options &fortranOpts() const { return parserOpts_; }
 
   TargetOptions &targetOpts() { return targetOpts_; }
-  const TargetOptions &TargetOpts() const { return targetOpts_; }
+  const TargetOptions &targetOpts() const { return targetOpts_; }
 
   Fortran::semantics::SemanticsContext &semanticsContext() {
     return *semanticsContext_;