.Net: Minor Documentation Corrections in MathPlugin: (#3657)

- Corrected the summary comment of the Subtract method to accurately
reflect its subtraction functionality.
- Added a point and fixed capitalization in the summary comment of the
Add method.

These changes improve the consistency and accuracy of the documentation
in MathPlugin.

### Motivation and Context

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### Description

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### Contribution Checklist

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- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

Co-authored-by: SergeyMenshykh <68852919+SergeyMenshykh@users.noreply.github.com>

dotnet/src/Plugins/Plugins.Core/MathPlugin.cs

@@ -10,9 +10,9 @@ namespace Microsoft.SemanticKernel.Plugins.Core;
 public sealed class MathPlugin
 {
     /// <summary>
-    /// Returns the Addition result of initial and amount values provided.
+    /// Returns the addition result of initial and amount values provided.
     /// </summary>
-    /// <param name="value">Initial value to which to add the specified amount</param>
+    /// <param name="value">Initial value to which to add the specified amount.</param>
     /// <param name="amount">The amount to add as a string.</param>
     /// <returns>The resulting sum as a string.</returns>
     [KernelFunction, Description("Adds an amount to a value")]
@@ -23,9 +23,9 @@ public sealed class MathPlugin
         value + amount;
 
     /// <summary>
-    /// Returns the Sum of two provided numbers.
+    /// Returns the subtraction result of initial and amount values provided.
     /// </summary>
-    /// <param name="value">Initial value from which to subtract the specified amount</param>
+    /// <param name="value">Initial value from which to subtract the specified amount.</param>
     /// <param name="amount">The amount to subtract as a string.</param>
     /// <returns>The resulting subtraction as a string.</returns>
     [KernelFunction, Description("Subtracts an amount from a value")]