Fixed a type error

Change-Id: I77c44e23a521e8b4b2a7efdc0669ecc8446e72ee

src/component/tooltip/TooltipHTMLContent.ts

diff --git a/src/component/tooltip/TooltipHTMLContent.ts b/src/component/tooltip/TooltipHTMLContent.ts
index 3236c95..9559cb2 100644
--- a/src/component/tooltip/TooltipHTMLContent.ts
+++ b/src/component/tooltip/TooltipHTMLContent.ts
@@ -429,7 +429,7 @@
         if (isString(content)) {
             if (self.trustedTypes && self.trustedTypes.createPolicy) {
                 const policy = self.trustedTypes.createPolicy('TooltipHTMLFix2', {
-                    createHTML: (string) => string
+                    createHTML: (string: string) => string
                 });
                 const safeContent  = policy.createHTML(content) as unknown as string;
                 const safeArrow = policy.createHTML(arrow) as unknown as string;