Fixed minor spelling issues

I was reading through index.d.ts because I am going to try to eventually resolve the issue of matrix sum in a higher dimension returning another matrix.

types/EXPLANATION.md

@@ -20,7 +20,7 @@ Maintaining the TypeScript types is done manually. When adding a function, one h
 3. Add a static definition inside `export const {...} : MathJsInstance`
 4. Add a dependencies definition inside `export const {...} : Record<string, FactoryFunctionMap>`
 
-For exampe for the function `add`, we can have the following definitions:
+For example for the function `add`, we can have the following definitions:
 
 ```ts 
 // instance

types/index.d.ts

@@ -4523,7 +4523,7 @@ export interface MathJsChain<TValue> {
   /**
    * Parse an expression. Returns a node tree, which can be evaluated by
    * invoking node.evaluate();
-   * @param options Available options: nodes - a set of custome nodes
+   * @param options Available options: nodes - a set of custom nodes
    */
   parse(
     this: MathJsChain<MathExpression>,