Fix doc comment for exception::line_number() (#901)

The documentation comment for `exception::line_number()` in
`include/etl/exception.h` states falsely that the return type for
the function is a `const char*`.

It should state that the return type is `numeric_type`,
which is an `int`.

So change the documentation comment to reflect that the return type is
an `int`.

include/etl/exception.h

@@ -100,7 +100,7 @@ namespace etl
 
     //***************************************************************************
     /// Gets the line for the exception.
-    /// \return const char* to the line.
+    /// \return int as line number.
     //***************************************************************************
     ETL_CONSTEXPR
     numeric_type line_number() const