Fix typos

Bring .NET Framework target down to 4.5
Rename project in comment

csharp/src/Google.Protobuf.Test.TestProtos/Google.Protobuf.Test.TestProtos.csproj

 <Project Sdk="Microsoft.NET.Sdk">
 
   <!--
-    This CodeGen project is kept seperate from the original test project for many reasons.
-    It allows us to make sure code can compile on a seperate compiler version, different frameworks,
+    This TestProtos project is kept separate from the original test project for many reasons.
+    It allows us to make sure code can compile on a separate compiler version, different frameworks,
     and without the internal visibility from the test project (all of which have caused issues in the past).
   -->
   <PropertyGroup>
-    <TargetFrameworks>net451;netstandard1.0;netstandard2.0</TargetFrameworks>
+    <TargetFrameworks>net45;netstandard1.0;netstandard2.0</TargetFrameworks>
     <LangVersion>3.0</LangVersion>
     <AssemblyOriginatorKeyFile>../../keys/Google.Protobuf.snk</AssemblyOriginatorKeyFile>
     <SignAssembly>true</SignAssembly>