(grammar) Standardize useage of possesive form of "decryption materials"

[MatthewBennington]

"Decryption materials" refers to a set of materials, and is plural. In most cases when we refer to an element within the set, we use the phrase "the decryption material's [x]". This is inconsistent, as nowhere else is "decryption materials" singular.

There is place place where we use the phrase "the decryption meterials' [x]". This maintains the plural "materials", however resident grammar person @juneb suggests plural possessive should be avoided.

% grep -nr "material's" framework
framework/raw-rsa-keyring.md:158:- The output of RSA decryption is set as the decryption material's plaintext data key.
framework/structures.md:161:  included in this decryption material's [algorithm suite](#algorithm-suite).
framework/structures.md:174:included in this encryption material's [algorithm suite](#algorithm-suite).
framework/structures.md:214:  included in this decryption material's [algorithm suite](#algorithm-suite-1).
framework/structures.md:227:included in this decryption material's [algorithm suite](#algorithm-suite-1).
 % grep -nr "materials'" framework
framework/raw-aes-keyring.md:146:The keyring MUST attempt to serialize the [encryption materials'](structures.md#encryption-materials)
framework/raw-aes-keyring.md:177:The keyring MUST attempt to serialize the [decryption materials'](structures.md#decryption-materials)