Fix NuGet license expression casing (APACHE-2.0 -> Apache-2.0)

SPDX license identifiers are case-sensitive. 'APACHE-2.0' triggers
NU5124 warning during dotnet pack. Corrects to the valid SPDX
identifier 'Apache-2.0'.

Author: CritasWang

src/Apache.IoTDB/Apache.IoTDB.csproj

@@ -6,7 +6,7 @@
         <Authors>eedalong, lausannel, MysticBoy, Aiemu, HTHou, CritasWang</Authors>
         <PackageProjectUrl>https://github.com/apache/iotdb-client-csharp</PackageProjectUrl>
         <RepositoryUrl>https://github.com/apache/iotdb-client-csharp</RepositoryUrl>
-        <PackageLicenseExpression>APACHE-2.0</PackageLicenseExpression>
+        <PackageLicenseExpression>Apache-2.0</PackageLicenseExpression>
 	      <RepositoryType>git</RepositoryType>
         <PackageReadmeFile>README.md</PackageReadmeFile>
         <Description>