Update Math.xml (dotnet#7492)

Typing error in Note: availble -> available

Author: RVerheggen

xml/System/Math.xml

@@ -5801,7 +5801,7 @@ The <xref:System.Math.Round%2A> method supports two rounding conventions for han
    Midpoint values are rounded to the nearest even number. For example, both 3.75 and 3.85 round to 3.8, and both -3.75 and -3.85 round to -3.8. This form of rounding is represented by the <xref:System.MidpointRounding.ToEven?displayProperty=nameWithType> enumeration member.
 
 > [!NOTE]
-> In .NET Core 3.0 and later versions, three additional rounding strategies are availble through the <xref:System.MidpointRounding> enumeration. These strategies are used in all cases, not just for midpoint values as <xref:System.MidpointRounding.ToEven?displayProperty=nameWithType> and <xref:System.MidpointRounding.AwayFromZero?displayProperty=nameWithType> are.
+> In .NET Core 3.0 and later versions, three additional rounding strategies are available through the <xref:System.MidpointRounding> enumeration. These strategies are used in all cases, not just for midpoint values as <xref:System.MidpointRounding.ToEven?displayProperty=nameWithType> and <xref:System.MidpointRounding.AwayFromZero?displayProperty=nameWithType> are.
 
 Rounding away from zero is the most widely known form of rounding, while rounding to nearest even is the standard in financial and statistical operations. It conforms to IEEE Standard 754, section 4. When used in multiple rounding operations, rounding to nearest even reduces the rounding error that is caused by consistently rounding midpoint values in a single direction. In some cases, this rounding error can be significant.