Update content-security-policies.mdx (#17677)

Rules can't modify CSP header, only add/replace or remove

src/content/docs/fundamentals/reference/policies-compliances/content-security-policies.mdx

@@ -28,7 +28,7 @@ Cloudflare does not:
 
 If you require the CSP headers to be changed or added, you can change them using some Cloudflare products:
 
-- If your website is [proxied](/dns/manage-dns-records/reference/proxied-dns-records/) through Cloudflare, you can use a [Response Header Modification rule](/rules/transform/response-header-modification/) to modify or add CSP headers.
+- If your website is [proxied](/dns/manage-dns-records/reference/proxied-dns-records/) through Cloudflare, you can use a [Response Header Modification rule](/rules/transform/response-header-modification/) to replace or add CSP headers.
 - If your website is hosted using [Cloudflare Pages](/pages/), you can set a [`_headers file`](/pages/configuration/headers/) to modify or add CSP headers.
 
 ### Product requirements