Update IronPython.nuspec (#1599)

* Update IronPython.nuspec

`System.Memory` was updated from 4.5.4 to 4.5.5 because of
#1562
https://github.com/IronLanguages/ironpython3/pull/1565/files#diff-c3abea1468e0811ae92a67f8e579a269327fd775ed5c273d2e5824366ad45a9a
This change is to align the version in nuspec

* Update IronPython.nuspec

Author: scott-xu

Package/nuget/IronPython.nuspec

@@ -20,11 +20,11 @@ This package contains the IronPython interpreter engine.</description>
     <dependencies>
       <group targetFramework="net462">
         <dependency id="DynamicLanguageRuntime" version="1.3.2" />
-        <dependency id="System.Memory" version="4.5.4" />
+        <dependency id="System.Memory" version="4.5.5" />
       </group>
       <group targetFramework="netstandard2.0">
         <dependency id="DynamicLanguageRuntime" version="1.3.2" />
-        <dependency id="System.Memory" version="4.5.4" />
+        <dependency id="System.Memory" version="4.5.5" />
         <dependency id="System.Text.Encoding.CodePages" version="4.6.0" />
         <dependency id="Microsoft.Win32.Registry" version="4.5.0" />
         <dependency id="Mono.Unix" version="7.1.0-final.1.21458.1" />